Electromagnetic induction has long been used for detection of hidden and buried metal objects such as mines and buried ordinance and for security checks, treasure hunting, and earth resource exploration. The method involves generating a primary electromagnetic field from a transmitter coil that induces a current in nearby conductive objects. This induced current produces a relatively weak secondary electromagnetic field that is detected by a receiver coil. The measured secondary electromagnetic field is then analyzed to detect, and sometimes characterize, buried or hidden objects.
Existing metal detectors employ many well-known transmitter and receiver coil configurations. One consideration in coil configuration is that the primary electromagnetic field generated by the transmitter coil not only induces a secondary electromagnetic field in the hidden object but also induces an electromagnetic field in the receiver coils. Because the object's induced secondary electromagnetic field is relatively weak compared to the primary electromagnetic field, the primary electromagnetic field will tend to mask the secondary electromagnetic field unless compensated for. This compensation is referred to as nulling or balancing. Ideally, nulling the primary electromagnetic field results in the metal detector indicating a field strength of zero, or null, in free space.
One approach to nulling is to employ multiple bucking coils. As shown in the patent to Won, U.S. Pat. No. 5,557,206, the disclosure of which is expressly incorporated herein by reference, another approach is to concentrically locate the receiver coil within the transmitter coils such that the direct electromagnetic field is automatically nulled.
While the approaches above present methods for measuring electromagnetic field strength, none measure electromagnetic field gradients. The measurement of electromagnetic field gradients is useful in characterizing hidden objects and provides another tool for determining the size, shape, or composition of the objects. Thus, there remains a need for method of measuring electromagnetic field gradients.